Metal plating is a well-known process employed to alter the existing surface properties or dimensions of a substrate. A substrate may be plated for decorative purposes, to improve resistance to corrosion or abrasion, or to impart desirable electrical or magnetic properties to a substrate.
There are various methods of plating, including electroplating and electroless plating. Electroless plating involves the deposition of a metallic coating from an aqueous bath onto a substrate by a controlled chemical reduction reaction which is catalysed by the metal or alloy being deposited or reduced. This process differs from electroplating in that it requires no external electrical charge. One positive feature of electroless plating over electroplating is the ability to plate a substantially uniform metallic coating onto a substrate having an irregular shape. Frequently, electroplating an irregularly shaped substrate produces a coating having non-uniform deposit thicknesses because of varying distances between the cathode and anode of the electrolytic cell. Another positive feature of electroless plating over electroplating is that electroless plating is autocatalytic and continuous once the process is initiate, requiring only occasional replenishment of the aqueous bath. Also, electroless coatings are virtually nonporous, which allows for greater corrosion resistance than electroplated substrates.
In general, an electroless plating bath includes water, a water soluble compound containing the metal to be deposited onto a substrate, a complexion agent that prevents chemical reduction of the metal ions in solution while permitting selective chemical reduction on a surface of the substrate, and a chemical reducing agent for the metal ions. Additionally, the plating bath may include a buffer for controlling pH and various optional additives, such as bath stabilizers and surfactants.
Numerous methods have been used to grow metal nanowires. Among all the methods, solution-phase synthesis is one of the most promising routes to prepare ID nanostructure in terms of cost, throughput and the potential for high-volume production. In Y. G. Sun, Y. D. Yin, B. T. Mayers, T. Herricks, Y. N. Xia, Chem. Mater. 14, (2002), 4736., Y. G. Sun, Y. N. Xia. Adv. Mater. 14, (2002), 833 and M. Giersig, I. Pastoriza-Santos, L. M. Liz-Marz'an, J. Mater. Chem. 14, (2004), 607 poly (vinyl pyrrolidone) (PVP), silver nanowires were synthesized by organic solvent reduction such as ethylene glycol and N,N-dimethylfonnamide (DMF). Silver nanowires also had been synthesized in aqueous solutions. For instance, Murphy and co-workers in N. R. Jana, L. Gearheart, C. J. Murphy, Chem. Commun. 7, (2001), 617, had reported a process to synthesize silver nanorods by reducing AgNO3 with ascorbic acid. Zhang et al. also had reported a seed-less synthesis of silver nanowires using ascorbic acid as reducer in the presence of poly (methacrylic acid) (PMAA). In J. Q. Hu, Q. Chen, Z. X. Xie, G. B. Han, R. H. Wang, B. Ren, Y. Zhang, Z. L. Yang, Z. Q. Tan, Adv. Funct. Mater. 14, (2004), 183, Tan and co-workers had synthesized silver nanorods and nanowires by reduction of AgNO3 with tri-sodium citrate in the presence of dodecylsulfonate.
U.S. Pat. No. 6,387,542 also teaches a process for the electroless plating of silver onto a substrate using an aqueous plating bath comprising silver nitrate, ammonium hydroxide, ammonium carbonate and/or bicarbonate and hydrazine hydrate as a reducing reagent. The composition of the plating bath allows metallic silver to be precipitated from the plating bath by boiling in the form of sediment uncoated surface. There is therefore a long unmet need for improved metallized optical fibers.